Joint compounds, also known as tape joint cements, are used in the application of wallboard to fill the spaces between adjoining boards so as to provide a smooth seamless surface. The same joint compounds are used to cover the nails or other fasteners which affix the wallboard panel to its support elements so as to conceal the nail head and impart a continuously smooth appearance to the board.
Emulsions are commonly used as binders for ready-mixed (wet) joint compounds. Polyvinyl alcohol stabilized vinyl acetate-ethylene (VAE) copolymers and hydroxyethyl cellulose stabilized vinyl acetate homopolymers are the two most common types. The emulsions which often are the most costly portion of the formulation, play a key role in determining the performance properties of finished joint compounds. The properties, such as adhesion, viscosity and crack resistance, are controlled to a large extent by the level and choice of the emulsion binder.
A common problem encountered by workers applying the Joint compounds is viscosity instability. A Joint compound will build structure or "gel" within hours after production. The compound becomes highly thixotropic requiring vigorous hand or mechanical stirring to make the joint compound flow for application. At this point, a second viscosity-related problem is observed. It is hoped that with sufficient stirring, the viscosity of the joint compound will return to the value at which it was manufactured. However, an applicator may open a container produced several months in the past and find that the agitated viscosity has either decreased such that sag on application is noted, or increased such that water must be added to lower viscosity. Water addition will increase shrinkage and/or promote shrinkage cracking. Either condition will generally slow the work or require rework, resulting in higher application costs. As a result, a joint compound that does not require agitation before use and does not change in viscosity over time would be a strong economic benefit.
In addition, there is currently no emulsion binder that is able to impart high efficiency, excellent application properties and viscosity stability all at the same time. Polyvinyl alcohol stabilized vinyl acetate-ethylene copolymer emulsions offer excellent bonding efficiency at ambient and low temperatures, but they impart marginal application properties (trowelability), high air entrainment (low density) and thixotropy build upon aging. Vinyl acetate homopolymer-type binders (plasticized or unplasticized) impart improved application properties that suffer from reduced adhesion efficiency. They also exhibit poor low temperature binding characteristics and thixotropy build upon aging. Thus, it would be an economic advantage for both the applicator and manufacturer to use an emulsion binder which imparts excellent application properties, high viscosity, high ambient and low temperature binding efficiency and excellent viscosity stability to the joint compound formulation.
Attempts have been made to improve application properties or reduce air entrainment caused by polyvinyl alcohol stabilized VAE emulsions in the joint compound. It has been shown that it is extremely difficult to improve a single property without adversely affecting a second or third property. For example, joint compounds have been made with hydroxyethyl cellulose stabilized VAE copolymer emulsions with improved application properties, but sacrificed efficiency and viscosity stability. Air entrainment was reduced by addition of nonionic and anionic wetting agents but adhesion and trowelability were adversely affected.
The following references are representative of the state-of-the-art:
U.S. Pat. No.3,891,453 discloses a joint compound which is preferably asbestos-free, for use in finishing joints between wallboards, comprising a filler, a binder, and two or all three of a water-holding agent; a slip-inducing colloid; and a non-swelling clay having sufficient pseudoplasticity to render the composition non-leveling.
U.S. Pat. No.4,454,267 discloses a lightweight joint compound for use in finishing joints between gypsum wallboards, comprising a filler, a binder, a specially treated expanded perlite, a non-leveling agent, and a thickener.
U.S. Pat. No. 4,629,751 discloses a gel-resistant composition especially suited for filling, patching, or repairing plaster, gypsum wallboard panels and the like comprising a blend of glass microbubbles containing borate, high molecular weight polyhydroxy compound such as polyvinyl alcohol functioning as a binder, water, and low molecular weight polyhydroxy compound in which at least two of the hydroxyl groups are attached to carbon atoms that are separated by one or more intervening carbon atoms.
U.S. Pat. No. 4,686,253 discloses expanded perlite coated with a composition to render the perlite impervious to water, and a joint compound incorporating the same.
EP 0,245,718-A2 discloses joint compounds prepared using as binders therefor vinyl ester polymers prepared by conventional emulsion polymerization techniques employing, as the stabilizer therein, an acid converted or enzyme modified starch having water fluidity of 10 to 85.